Linear hydroxyaryloxy-functional siloxanes are useful starting materials for making polydiorganosiloxane/polyorgano block copolymers. There are three general pathways known to prepare linear hydroxyaryloxy-functional siloxanes.
U.S. Pat. No. 3,189,662 describes the reaction of chloro-terminated polysiloxanes with bisphenolic compounds, eliminating hydrochloric acid as the byproduct. This process has the disadvantages of requiring the use of large amounts of a basic compound to neutralize the hydrochloric acid byproduct and a tedious filtration of the resulting salt.
U.S. Pat. Nos. 4,584,360 and 4,732,949 describe the reaction of bisphenolic compounds with α,ω-bisacyloxypolydimethylsiloxanes, which are represented by a structural formula of HO—Ar—O—(SiR2—O)o—(SiR2—O)p—(SiR12—O)q—Ar—OH, where Ar are arylene radicals from diphenols, R and R1 are alkyl or aryl and o+p+q is from 5 to 100, in a molar ratio of 2:1 to 20:1 in an inert organic solvent using at least one inorganic base in at least stoichiometric amounts. According to the '360 and the '949 patents, the preferred inorganic bases are alkali metal and alkaline earth metal carbonates.
In order to dissolve the large excess of bisphenolic compounds used, the process disclosed in the '360 and the '949 patents requires the use of large amounts of organic solvents, typically chlorinated organic solvents. Use of these chlorinated organic solvents in large amounts is not desirable for health, safety and environmental concerns. And removal of the large amounts of solvents by distillation increases manufacturing costs. Furthermore, the base used in the reaction mixture forms salts, which are difficult to be completely removed from the hydroxyaryloxy-terminated siloxane product by filtration. Thus, isolation of hydroxyaryloxy-terminated siloxanes according to this process in a pure form which is free of undesirable impurities is tedious and costly.
U.S. Pat. No. 6,258,968 describes the reaction of bisphenolic compounds with cyclodialkylsiloxanes in a solvent, whereby an acid catalyst is used and the byproduct water is removed from the reaction mixture by distillation. This process has a number of disadvantages. Firstly, the process is limited to simple monocyclic bisphenols, such as hydroquinone, as bicyclic bisphenols such as bisphenol-A decompose under acid catalysis, forming numerous undesired side-products. Secondly, it is difficult to control the structure of the hydroxyaryloxy-functional polysiloxane products prepared by this process, given that the molecular weight of the product is determined by the exact amount of water removed and the reactivity of the bisphenol. Removal of too little water leads to incomplete reaction and the formation of undesired terminal Si—OH groups, whereby removal of too much water yields polymers of excessively high molecular weight and viscosity. It has been found extremely difficult to obtain polymers free of terminal Si—OH groups that are of sufficiently low viscosity for easy filtration. Filtration of high viscosity polymers without applying heat is slow and tedious, adding significantly to the cost of such processes. Thirdly, the salts formed after neutralization of the acid catalyst have been found to be extremely difficult to remove by filtration, especially if there is unreacted bisphenol in the mixture.
Hydroxyaryloxy-terminated siloxanes can be used to prepare polydiorganosiloxane/polycarbonate block copolymers via a two-phase boundary process or a solventless polycondensation, transesterification, or melt process. Since the solventless polycondensation, transesterification, or melt process does not allow for a subsequent purification step, it is particularly sensitive to impurities. Residual byproducts and impurities that cannot be removed from the hydroxyaryloxy-terminated siloxanes, such as neutralization salts, can be detrimental to the properties of the resulting block copolymers. For example, such impurities can cause haziness and surface defects in molded parts, and reduce the stability towards hydrolysis and chemicals.
Heretofore, it is not believed to be possible to prepare hydroxyaryloxy-functional siloxanes of controlled structure and free of undesired Si—OH and other side products by the prior art processes discussed above in a reproducible and cost effective manner. Furthermore, the hydroxyaryloxy-functional siloxanes prepared by the prior art processes discussed above are typically contaminated by residual neutralization salts and excess bisphenolic compounds. These hydroxyaryloxy-functional polydiorganosiloxanes, when used to prepare polydiorganosiloxane/polyorgano block copolymers by solventless polycondensation, transesterification, or melt processes, may cause haziness or impair the thermal and chemical stability of the copolymer product.
Accordingly there is a need for a cost effective process for the preparation of linear hydroxyaryloxy-terminated polydialkylsiloxanes having controlled structure that are free of unwanted impurities.